It doesn't let scientists turn genes on and off at will. Firstly, it doesn't work for all genes, and even for the genes it works for it doesn't turn them off completely but rather lowers their activity significantly. They're also considerably worse at turning genes back on after turning them off. Additionally, it's really difficult to actually activate this system. They're working in cultured cell lines it hasn't even been tested in mice or anything related, in fact, editing genes in a mature organism is almost impossible with current technology.
They also didn't try and turn on any genes they haven't previously turned off which is another question.
This technology significant for science, but it's not necessarily a landmark. The first paper to epigenetically silence genes using modified CRISPR/dCas9 via DNA methylation was published in 2016.
Editing DNA methylation in the mammalian genome
I think it's important to watch out for these sensationalized headlines.